This invention relates to internal combustion engines and the disposition of ancillary components, and more particularly to coolant system heat exchanger location for inverted engines for aircraft propulsion.
The term xe2x80x9cradiatorxe2x80x9d is used herein to embrace any form of heat exchanger and is not restricted to a particular form or mode of heat transfer; in fact, radiation is not a significant mode compared with conduction and convection. A particular example is a liquid/air radiator, with a honeycomb matrix of vanes (of large collective surface area) about a convoluted network of tubular flow passages between a supply tank and a collection tank. Heat transfer is primarily by conduction and (forced) convection to a (generally forced) air flow across the vanes and flow passages.
The diversity of multi-cylinder configurations for aircraft piston enginesxe2x80x94typically with a single, common crankshaftxe2x80x94include, for example: a single-file row (i.e. an xe2x80x9cin-linexe2x80x9d configuration); multiple, discrete, angularly-splayed, or angularly offset, rows (albeit there may be only one cylinder in each row)xe2x80x94such as a xe2x80x9cVxe2x80x9d or xe2x80x9cWxe2x80x9d configuration; in rows opposed, either horizontally, vertically, or at some other angle (e.g. a flat configuration); and individually, around a common crankshaft axis, generally equi-angularly spaced, in one or more planes (e.g. a xe2x80x9cradialxe2x80x9d configuration). There also have been some engines with multiple crankshafts, for example, with cylinders arranged in an xe2x80x9cHxe2x80x9d configuration (in effect, two flat engines sharing a single common crank-case), or with two pistons per cylinder, working in opposition, in various opposed-piston arrangements.
It is known to xe2x80x9cinvertxe2x80x9d an in-line, or xe2x80x9cVxe2x80x9d configurationxe2x80x94so that the cylinders are below the crankshaft. A prime advantage of such engine inversion, for aircraft propeller propulsion, is that the crankshaft sits higher on the engine and so a propeller mounted directly upon it will be farther from the ground. At critical flight phases of take-off and landing, it is important to maintain adequate clearance between propeller and ground. The object is to reduce the chance of accidental propeller damage, allowing for undercarriage travel and fuselage forward tipping moment about the undercarriage.
Other ways to improve ground clearance include lengthening the undercarriage, in order to raise the whole aircraft further from the ground, reducing the diameter of the propeller, and raising the engine installation in the aircraft. All of these have drawbacks, however. It is thus well-established for smaller aircraft that use directly-driven propellers (i.e. propellers mounted directly upon a crankshaft end), to use an inverted engine arrangement.
Waste heat from an internal combustion (IC) engine has to be transferred to its surroundings, in one way or another.
For most engines, (except in marine use) the only convenient way to dispose of this heat is to transfer it to the surrounding air. Such heat transfer can be directly from the engine components (i.e. xe2x80x9cair-coolingxe2x80x9d) in which case the components are usually made with fins to provide a greater surface area for heat transfer by conduction and convection.
Heat transfer can also be through an intermediate fluid, such as oil, water, and/or ethylene glycol, circulated around the various parts of the engine in order to collect heat then passed to a heat exchanger (xe2x80x9cradiatorxe2x80x9d) , where the heat is transferred to the air. The extra complexity of providing an intermediate fluid for cooling is a disadvantage, but it enables a reduction in the temperatures of key components, thus allowing a given size engine to be made more powerful, more reliable and longer-lasting. It is essential that the cooling system be made extremely reliable, since engine componentry that is not effectively cooled will overheat and fail rapidly.
The heat exchanger (or radiator) usually comprises a series of finned tubes and fluid collectors at each end of these tubes. The fins provide the large surface area required for transfer of the heat, by convection to the air. The radiator may be made in discreet sections (each of which may comprise a number of tubes and their associated fins), which are then assembled into a single unit.
Typically a fan, or multiple fans, are used to increase the velocity of the air over the fins of the radiator, hence improving the heat transfer coefficient and allowing a smaller radiator to be used. For vehicles, the movement of the vehicle may be sufficient to provide the relative air velocity, although a fan, or multiple fans, are often used as well.
In the specific case of aircraft engines, the velocity of the aircraft once flying is usually sufficient that a fan is not necessary. The air-displacement, thrust action of a propeller itself provides a very convenient high velocity flow of air that can easily be used to advantagexe2x80x94especially when the aircraft is stationary on the ground, or has a low airspeed when climbing.
The engine lubricating oil is not usually the primary coolant, but often becomes hot, because of its contact with the high temperature components in the heart of the engine and the frictional heat that is generated at various component sliding contact surfaces. Engine oil is thus often cooled by its own dedicated cooler which may transfer the heat directly to the air, in a heat exchanger radiator or to an intermediate fluid, and thence to the air.
Many different locations for coolant and oil cooling radiators have been adopted. While a lubricant (oil) radiator is generally smaller, and is often mounted to the engine assembly, a coolant radiator is usually mounted elsewhere upon the airframe, for example, under the fuselage, inside the fuselage, under, or inside, the wing structure, etc. Smaller inverted aircraft engines have often been air-cooled, with no requirement for a coolant radiator. Where a lubricant (oil) radiator has been used, it is generally mounted towards the rear of the engine, or upon the airframe remote from the engine.
Some flat (horizontally opposed) engines have used oil radiator locations below and beside the engine crankshaft axis, and toward the front of the engine. Larger inverted engines have been liquid-cooled, but with radiators for coolant and lubricant (oil) mounted upon the airframe, remote from the engine.
According to the present invention, a fluid coolant heat exchanger, such as a radiator matrix or honeycomb, is mounted directly upon an engine or engine casing, at a location below a crankshaft axis, of an inverted internal combustion (IC) engine. In practice, the coolant fluid is a liquid, conveniently water, albeit with corrosion and freezing inhibitor agents, such as alcohol, or ethylene glycol. Alternatively, for severe low-temperature duty an entirely synthetic coolant may be employed.
Such a radiator location is conveniently adopted with special, or dedicated engine features, for example, a (forward) extension of the crankshaft and crankcase (nose), to allow room for the radiator and associated airstream. Although this involves some additional engine cost and complexity, considerable benefits accrue to an integrated engine cooling system which have surprisingly been found to outweigh apparent disadvantages.
The radiator can be used for cooling either coolant, lubricant (oil), or both (on a combined unit) so the heat exchanger location is applicable to either a primarily liquid or air-cooled engine. The radiator can be conveniently attached to the engine structure, either directly, or indirectly, by compliant mountings, that help prevent, absorb, or suppress, transmission of (potentially) damaging vibration from the engine structure.
The location according to the invention allows conveniently short and direct connection of fluid lines (if required) between engine and radiator with the benefits of reduced cost, installation time and skill and reduced risk of leaks. For aircraft, risk reduction is of paramount importance. Further, it allows direct passage of cooling air, from behind a propeller xe2x80x9cdiskxe2x80x9d, through the radiator without the need for additional ducting.
The radiator can desirably have extensions, in order to make use of any available space around the front of the engine with bespoke complementary profiling to fit around other components and auxiliaries.
With a radiator mounted directly upon the engine structure, with or without compliant mountings, direct connecting passages can be used thereby eliminating hoses, other fittings, or connections, with reduction in cost, installation time, and risk of fluid leakage.
On xe2x80x9cpusherxe2x80x9d aircraft types, where the propeller is at the rear of the engine, air can first be ducted into the engine compartment and be exhausted through the radiator for onward flow through the propeller disk.